Germinal center

As a germinal center is defined as the costs incurred after antigen contact inside a Lymphfollikels reactive changes, which thus becomes a secondary follicle. The germinal center is a light- microscopically visible topographical separation of different developmental stages of B cells that could lead to immunological processes. The dark inner zone contains proliferating activated B cells ( centroblasts ). These migrate as centrocytes from the outside light zone and are selected on the edge. This is followed by a coat and marginal zone. In the latter, the memory B cells develop.

Nucleation centers are referred to as "risk zones" since many lymphomas derived from B lymphocytes.

Immunological processes before germinal center formation

B cells to bind with their B-cell receptors, a native soluble peptide antigen will be included together with the receptors bound antigens in the cell. The antigens are proteolytically cleaved and bound as peptide fragments 13 to 17 amino acids at MHCII molecules. These MHCII -peptide complexes are presented on the cell membrane.

These B - cells then reach the T-cell areas of secondary lymphoid organs. There are possibly for the same antigen -specific, already activated T cells. They recognize their T-cell receptors presented by the B cells MHC II -peptide complexes. Press and hold the B cells determines activate them and form them in the T cell zone is a primary focus in the proliferating B cells.

Some of the descendants of these B cells forms two to three days after activation Extrafollicular foci (eg in the red pulp of the spleen or in the medullary cords of the lymph nodes ), where they proliferate and differentiate into plasma cells. They secrete the foci IgM and IgG with low antigen affinity. Most are short-lived, and have a 50 % survival rate of three to five days. The foci disappear after about 14 days.

The other part of the B cells migrate along with the T- cells from which they were enabled in the B cell follicles, where they still form on the border of T-cell zone of germinal centers.

Emergence of a germinal center

With the emergence of a germinal center - germinal center reaction - created first a dark zone in the activated B cells ( centroblasts ) are subject to a massive clonal expansion. In this case, a class change of the constant region of the heavy chain held and somatic mutations in the genes for the variable regions of the light and heavy chains of the immunoglobulins are added. This can lead to an increase or decrease in the affinity of the B cell receptor to its antigen.

B-cells then migrate out of the dark zone out into the network of follicular dendritic cells and form the outer, light zone of germinal center. The impression of a " bright zone" in the histological picture is created by the fact that large basophilic nuclei B- immunoblasts arise from the B cells that migrate from the periphery to the center.

The B cells ( centrocytes ) no longer divide. The follicular dendritic cells present immune complexes of complement, antibodies and antigens, which they have bound via Fc and complement receptors. If the receptor affinity of centrocytes to antigen is too low, they can not bind to the antigen presented with their receptors. Is their affinity is not too small, they can bind these antigens and involve, reduce and express the peptide fragments bound to MHC II on the cell surface.

The centrocytes then migrate to the edge of the light zone of the germinal center, where they meet the also to some extent clonally propagated CD4 T cells, where they present the MHCII -peptide complexes. Here is a selection takes place: The B- cells that have gained a lower affinity and present it to T cells no antigen, no survival signal - they die apoptotic. Only B cells whose affinity is high enough to the original antigen and added antigens and present it to T cells, receive survival signals.

You either develop into plasma cells or memory cells. The development of plasma cells takes place on the stage of plasma blasts. Plasma blasts are still capable of division, but can already secrete antibodies. Plasma cells secrete antibodies, but are no longer capable of division. The development to memory B cells takes place in the marginal zone. You can also return to the dark zone of the germinal center and again subject to a germinal center reaction. Much of the resulting plasma cells in germinal centers migrate into the bone marrow, where they secrete the antigen with high affinity antibodies of classes IgG, IgE and IgA. Moreover, they are durable (some longer than one year).

During the whole process of germinal center formation, circulating through the B- cell follicles, B cells are pushed more and more to the outside and form a germinal center, the surrounding mantle zone.